The Court is powerful and important. Its ability to strike down state and federal laws means that it sets the government's boundaries. It also plays a leading role in articulating America's constitutional commitments. It gives substance and definition to the majestic generalities of the Constitution, phrases such as "due process" and "equal protection." In a real sense, the subject of Supreme Court decisions shows who we are as a people, what values we hold dear.

With so much power concentrated in the hands of so few, citizens must be able to evaluate the court's performance.

Activism vs. fidelity is flawed

Conventional wisdom focuses on a distinction between what we could call activism and fidelity. Faithful judges (the good ones) apply the law regardless of their own views. Activist judges (the bad ones) rule based on their own preferences.

This model of activism and fidelity is the one Chief Justice John Roberts invoked in his 2005 confirmation hearings when he promised to be an umpire, not a player.

Unfortunately, it is useless in evaluating decisions because it offers unrealistic caricatures on both sides. True activists don't exist; all judges believe that they are faithfully applying the law. But objective umpires don't exist either, because the Constitution does not provide clear answers in hard cases. That is what makes them hard. Consider some of the most controversial decisions from the just-concluded term.

Does the Constitution's protection of the freedom of speech mean that Congress cannot regulate corporate political advertisements that in effect endorse or oppose particular candidates? Does it mean that school officials cannot regulate off-campus speech by students? Does the guarantee of equal protection mean that school boards cannot take an individual's race into account in assigning students to public schools in order to promote racial integration? And how does the due process clause apply to a ban on partial-birth abortion?

Anyone who is candid about these questions will admit that the Constitution itself does not tell courts how to decide them. The Constitution indicates that some values – speech, equality, and liberty – are important. But it does not explain how to balance those values against competing government interests, or even how, precisely, these values should be understood.

Nor, in fact, does the Constitution say that the task of balancing values and interests is given to judges alone. Other people can balance, too, perhaps better than the court. Maybe Congress is better at figuring out what regulations will mitigate the corrupting influence of money on politics. Maybe the president is better at deciding what national security requires. Maybe school administrators are better at deciding what measures will fulfill their educational missions.

The key question is not whether judges are activist or faithful. It is when the court should be assertive in enforcing a constitutional provision, disregarding the views of others, and when it should be deferential, respecting their views. Understanding that the key choice is the one between deference and assertiveness takes us away from useless rhetoric about activism and back to basic principles of American government.

Judicial review is the court's contribution to the separation of powers, its role as check and balance in our system of divided government. The question we should ask is when do we want the Court to be a meaningful check on the president and Congress, a meaningful supervisor of the states? And when do we want the political branches to have the last word?

With this question in mind, we can evaluate the court's performance by asking whether its choices of deference and aggression can be explained by any principle. Recent decisions show no obvious pattern.

In upholding the federal partial-birth abortion ban, the court did defer to Congress, but not in the ordinary sense of allowing legislators to make a choice between competing values such as women's liberty and fetal life.

Instead, it elevated the medical judgment of members of Congress above that of doctors, including the American College of Obstetrics and Gynecologists, which held that the banned procedure was sometimes the safest way to terminate a particular pregnancy.

That was a strange choice. As the Terry Schiavo case showed, Congress is not very good at playing doctor in politically charged areas.

Nor do recent rulings indicate a pattern of deference to Congress. In an area where Congress has more relevant expertise – the conduct of political campaigns and elections – the court did not defer.

It struck down part of the Bipartisan Campaign Reform Act that restricted the ability of corporations to oppose specific candidates at election time, even though it agreed that Congress could attempt to limit the corrupting effect of money on politics. It differed with Congress as to whether this restriction was an appropriate method of doing so, and it followed its own judgment.

Is there a pattern of protecting free speech? No; in the so-called Bong Hits 4 Jesus case, the court allowed public school administrators to punish a student for his speech, even though the speech occurred off-campus and did not disrupt a school event. Speech that promotes drug use, the court said, is inimical to education.

Is there perhaps a pattern of deferring to experts or local authorities such as school boards? No; the court rejected expert medical views in the partial-birth abortion ban case, and it rejected the views of school boards in two cases dealing with attempts to foster integration in public schools in Seattle and Louisville, Ky.

The court did not say that there was anything wrong with the goal of integration. But because the school administrators had chosen to attain integration by using a system that sometimes considered race in student assignment, the court held that they had violated the Constitution. Not only did the court not defer to the judgment of school administrators, it declared that what they had done was constitutionally just as suspect as the South's segregation after the abolition of slavery.

A mirror for society

How can we explain these decisions? The absence of a pattern suggests that what might look like deference is actually agreement. The Roberts court does not leave issues to the political process when it thinks that others are better at balancing. It upholds government actions only when it thinks the government has gotten the constitutional balance right.

What drives the decisions is not a theory about when deference is appropriate but simply the court's view of the relevant constitutional provisions. This across-the-board assertiveness makes those views especially important.

The Roberts court does not believe that the due process clause provides much protection for the legal right to an abortion. It seems to favor corporate electioneering over student speech. And it believes that all government use of race offends the equal protection clause, whether it is done to segregate or to integrate. Equality, in this view, is not threatened by public schools coming to mirror residential patterns of racial segregation, but rather by the government considering race in trying to promote integration.

These are judgments about America's core constitutional values, about who we are as a people. Such rulings are not required by the words of the Constitution; indeed, for the past 40 or 50 years, Americans have lived with dramatically different interpretations of constitutional law. The court's decisions are holding a mirror up to society. To evaluate its performance, we need only ask if we recognize ourselves.

• Kermit Roosevelt is a professor of law at the University of Pennsylvania and the author of "The Myth of Judicial Activism."